1. Field of the Invention
The present invention relates to a multi-antenna communication apparatus for use in a wireless system for simultaneously receiving signals by a plurality of antennas, and controlling directivity or performing high speed transmission utilizing spatial correlation reduced in a certain radio wave environment.
2. Description of the Background Art
In recent years, widespread are various applications, for radio communication, used for a mobile telephone, a wireless LAN, a digital television, and the like. Therefore, a next-generation wireless system is required to solve the following two problems.
One of the problems is occurrence of fading. The fading occurs in an environment (referred to as multiple wave environment) in which radio waves are transmitted via a plurality of reflex paths from a certain antenna, and received by another antenna. In the multiple wave environment, a plurality of reflected waves are generated, and counteract each other when a certain positional relationship condition between antennas is satisfied. The plurality of reflected waves counteract each other because a phase difference between the radio waves transmitted via different paths is 180 degrees at a reception antenna side under the certain condition. Therefore, receiver sensitivity becomes substantially poor, thereby interrupting the communication.
The other of the problems is that it is necessary to increase frequency bands so as to realize high speed transmission. Advance in IT technology leads to substantial increase of data amount used in mobile networks. However, available radio frequencies are limited. As a result, a conventional communication system does not allow substantial increase of data transmission speed.
One of the wireless systems capable of solving the problems is a multi-antenna wireless system. The multi-antenna wireless system performs communication using a plurality of antennas. The multi-antenna wireless system allows a total directivity of the plurality of antennas to be electrically controlled, and allows selection of one of the plurality of reflected waves, thereby preventing the fading.
Further, in the multi-antenna wireless system, a plurality of antennas receive radio waves transmitted by another plurality of antennas, and separate received data into data corresponding to each of the transmission antennas. This system is called MIMO, which allows the data transmission speed to be enhanced.
FIG. 17 is a diagram illustrating an exemplary structure of a conventional multi-antenna wireless device disclosed in Japanese Laid-Open Patent Publication No. 2002-374224 (Patent Document 1). As shown in FIG. 17, in a conventional art, the number of reception circuits required for realizing the multi-antenna function is the same as the number of reception antennas. If this technology is applied to the mobile communication terminal, a scale of the terminal is increased, thereby reducing competitive advantages in market.
This problem is solved by using a compact multi-antenna wireless device disclosed in, for example, Japanese Laid-Open Patent Publication No. 62-279743 (Patent Document 2), Japanese translation of PCT international application No. 2004-521575 (Patent Document 3), and Japanese translation of PCT international application No. 2005-522909 (Patent Document 4).
FIG. 18 is a diagram illustrating an exemplary structure of the conventional multi-antenna wireless device disclosed in Patent Document 2. In an example shown in FIG. 18, frequencies of local oscillator signals used for two RF circuits are different by fs from each other. Therefore, IF frequencies are different by fs from each other. Accordingly, the local oscillator signals of the two systems are combined, and the combined local oscillator signals are demodulated in a circuit performing the following processes. Thus, a portion of an IF circuit and a portion of a demodulation circuit is shared by the two systems, thereby reducing a size of a reception circuit.
FIG. 19 is a diagram illustrating an exemplary structure of a conventional multi-antenna wireless device disclosed in Patent Document 3. In an example shown in FIG. 19, a mixer is provided following one of two antennas. Further, for a signal of a local oscillator of the mixer, used is a frequency which is different by fs from a harmonic frequency which is twice as high as an RF. Thus, it is possible to combine, with the RF, a frequency of a mixer output signal which is different by fs from the RF frequency, and down-convert and demodulate the combined signal. Thus, an RF circuit, an IF circuit, and an AD converter are shared, thereby reducing a size of a reception circuit.
FIG. 20 is a diagram illustrating an exemplary structure of a conventional multi-antenna wireless device disclosed in Patent Document 4. In an example shown in FIG. 20, modulators (coders) are provided following antennas, respectively. Each of the modulators modulates a reception signal of a corresponding one of the antennas by using an orthogonal code typified by Walsh code. Thus, the reception signals of a plurality of antennas having the same frequency are combined with each other, and a demodulation circuit can separate the reception signals. Therefore, an RF circuit and an AD converter can be shared by the antennas of all systems, thereby reducing a size of a reception circuit.
However, the conventional multi-antenna wireless device shown in FIG. 18 requires RF circuits of two systems. Further, the RF circuits of two systems have the local oscillator frequencies different from each other, and therefore the local oscillators of two systems are required. Therefore, reduction of an overall size of the multi-antenna wireless device is limited.
Further, the conventional multi-antenna wireless device shown in FIG. 19 requires a mixer and a local oscillator operating at the double harmonic frequency. Further, the conventional multi-antenna wireless device shown in FIG. 19 requires a filter for suppressing an interfering wave having a frequency which is different by fs from a desired frequency of a signal received by each of the antennas. Therefore, reduction of an overall size of the multi-antenna wireless device is limited.
Further, in the conventional multi-antenna wireless device shown in FIG. 20, it is necessary to provide a modulator operating at an RF for each of the antennas. Therefore, reduction of an overall size of the multi-antenna wireless device is limited.
Therefore, Japanese Laid-Open Patent Publication No. 2006-135814 (Patent Document 5) discloses a multi-antenna wireless device which is able to prevent the fading, perform high speed transmission without increasing a frequency band, and increasingly reduce a device scale as shown in FIG. 21.
However, in the conventional multi-antenna wireless device shown in FIG. 21, a multiplexer 20 switches from a certain antenna to another antenna too rapidly, whereby a discontinuous point of a signal appears. In this case, if a difference is large between a reception signal voltage of the certain antenna and the reception signal voltage of the another antenna to which the certain antenna has been switched, a voltage waveform becomes discontinuous when the switching is performed, and therefore an output signal contains large harmonic components. A waveform of the signal containing the harmonic components is distorted due to the frequency characteristic thereof when the signal containing the harmonic components passes through a matching circuit or a baseband filter circuit used in a radio frequency circuit, which may causes a transmission error.